The present invention relates to a two-component developer. More particularly, it relates to a developer comprising a carrier coated with a polyolefinic resin and a toner which manifests a favorable positive chargeability in its friction against of the carrier.
Hitherto known as an electrophotographic electrostatic latent image developing system is the two component type developing system in which an insulating toner is frictionally electrified by mixing the toner with carrier particles and the developer is carried to the position where it is to come in contact and develop the electrostatic latent image.
The granular carrier used for such a two component developing system is normally coated with some appropriate material for the purposes of prevention of filming of the toner on the carrier surfaces, formation of uniform carrier surfaces, prevention of its surface oxidation, prevention of reduction in its sensitivity caused by humidity, prolongation of the life of the developer, protection of the photosensitive material from scratches or abrasion by the carrier, control of polarity of electrification and adjustment of the rate of electrification and so on.
Carriers on which polyolefinic resins are applied as such coating materials are known (e.g., Japanese Patent Laid-Open Publication No. Hei 1-7255).
The polyolefinic layer which appears in the aforementioned patent publication is formed by direct polymerization on the surface of the carrier core and is therefore excellent in adhesiveness to the core, causing no deterioration in the picture quality even when the copying is continued long, and excellent also in durability and resistance to the phenomenon of being spent.
On the other hand the toner, being the other element of the two-component developer, is a black toner having carbon black as a coloring agent added to and dispersed in mainly a thermoplastic resin. The carbon black added functions mainly as a coloring agent, but also plays the role of a charging controller, taking advantage of its conductivity. Therefore, the role of the carbon black as an additive is important; particularly, its dispersibility in the toner has a large bearing on the quality of the copied image and ease of use of the toner. Thus if the dispersibility of the carbon black in the toner is low, the toner will not be electrified uniformly. Such a toner tends to scatter, causing stained interior of a copying machine or fogs on the ground of the copied image. Further, if the dispersibility of the carbon black is low, as the copying is repeated, the amount of electrification of toner fluctuates, causing deteriorated quality of the copied image.
Further, to the toner, in general, a charging controller is added for adjusting the charging level and for ensuring the electrification stability. For toners to be positively charged, a positive charging controller such as Nigrosine dyes are used.
However, the polyolefinic resins themselves tend to be negatively electrified and therefore the toner will be positively charged in excess, depending on the positive charging controller used. But if the positive charging controller is to be used in smaller amount in order to lower the charging level, uniform dispersion of the positive charging controller into the toner will become difficult, resulting in insecurity in stable electrification, scattering of the toner and so forth.
Further, the charging controller exposed or sticking to the toner surface adheres to and accumulates on the carrier surface, due to the frictional contact between the carrier and the toner by dint of a long time stirring. As a result, the carrier's intrinsic electrification characteristic is impaired, resulting in insecurity in electrification and thereby causing scattering of the toner and the like. This phenomenon is also brought about by the toner spent to the carrier.
As described above, normally, the charging characteristic of a developer is largely influenced by the way of combination of the toner and the carrier, so that it needs to be examined on their individual combinations.